1. Field of the Invention
This invention relates to polarization detectors and more particularly to polarization detectors for use in optical recording systems.
2. Description of the Prior Art
Polarized light is used in a number of applications such as thin film depth measurement and magneto-optic recording. In such systems it is necessary to be able to determine the state of polarization, i.e. direction of the plane of polarization.
Magneto-optic recording systems allow erasable storage of data. A laser beam is focused onto a spot on a magneto-optic disk medium and heats the material to a temperature at which the magnetic domain of the medium may be changed. A magnetic field is then applied in one of two directions and orients the magnetic domain of the spot in either an upward or downward direction.
The disk is read by focusing a low-power polarized laser beam onto the spot on the disk. The Kerr effect causes the reflected beam's plane of polarization to be rotated either clockwise or counterclockwise depending upon the direction of the magnetic domain of the recorded spot. The difference in the rotation is detected and represents the digital data. The disk can be rerecorded by heating the spots and switching the directions of the magnetic domains.
The detection of the direction of polarization is critical to these optical recording systems. Some of these systems use discrete optical components, such as Wollaston prisms, to separate the reflected light beam into two orthogonal polarization component beams. The intensity of these beams is then detected and compared. The exact direction of the plane of polarization of the original reflected beam can then be determined.
Another way to separate the orthogonal polarization components is to use optical grating couplers. Examples include U.S. Pat. No. 4,868,803 issued Sep. 19, 1989 to Sunagawa et al.; "Focusing Grating Couplers for Polarization Detection" by Shogo Ura et al., Journal of Lightwave Technology, Vol. 6, No. 6, June 1988; and "An Integrated Optic Detection Device for Magneto-optical Disk Pickup", by Sunagawa et al., International Symposium On Optical Memory 1987, September 16-18, 1987, Tokyo, Japan. These references show a combination of Transverse Electric (TE) and Transverse Magnetic (TM) grating couplers positioned side by side in the beam path of light returning from a magneto-optic disk. The TE and TM gratings each couple a different orthogonal polarization component into a waveguide. A problem with this approach is that only portions of the beam are being sampled for each polarization component. The beam is often nonuniform in its intensity, and errors in detection can occur.
What is needed is an integral polarization detection system with improved sensitivity and reliability.